Aluminum nitride has excellent mechanical properties, such as high hardness, a high elastic modulus and high stiffness, and also has excellent thermal and electric properties as a printed circuit board or a heat dissipation material, such as high thermal conductivity and a low thermal expansion coefficient while being electrically nonconductive. Accordingly, aluminum nitride is a compound drawing large attention in various industrial fields including electronic industries, as a hard material or a material for a part of semiconductor device processing equipment, and as a thermal management material.
As methods of synthesizing aluminum nitride, a method of synthesizing aluminum nitride by mixing aluminum oxide and carbon and heating the mixture thereof at a high temperature of 1,800° C. under a nitrogen gas atmosphere to induce reduction of aluminum oxide by carbon and a subsequent nitridation reaction between the reduced aluminum and a nitrogen gas (known as a carbo-thermal method), and a direct nitridation method in which aluminum powder and a nitrogen gas are reacted at a high temperature (known as a direct nitridation method) are industrially widely used. The present disclosure relates to a method of synthesizing aluminum nitride via a nitridation reaction of aluminum powder and a nitrogen gas.
According to a preceding study (P. Sthapitanonda, J. L. Margrave, “Kinetics of Nitridation of Magnesium and Aluminum”, J. Phys. Chem., vol. 60 (1956), pp 1628-1633), once a nitride layer is formed on a surface of aluminum powder at a low temperature, a rate of a subsequent nitridation reaction is controlled by diffusion of a reactant, and reaction kinetics of formation of nitride follows a parabolic law. In this regard, although formation of nitride may occur at a high rate in the earlier stage, once a core-shell structure where solid-phase un-reacted aluminum is covered with nitride is formed, the rate of formation of nitride decreases with time, and thus only a limited amount of nitride may be formed.
Therefore, for efficient nitridation, the nitridation reaction must proceed as a rapid combustion reaction rather than a reaction controlled by diffusion. One approach is a self-propagating high temperature combustion synthesis method, in which a part of an aluminum powder stack is intensively heated using an ignitor to initiate a rapid nitridation reaction and generate a large amount of exothermic heat, such that un-reacted aluminum continues to produce a high temperature exothermic synthesis reaction. Another approach is a volumetric thermal explosion method in which aluminum powder stack or porous aluminum powder compact is heated under a nitrogen gas atmosphere to cause volumetric combustion synthesis in a thermally explosive mode involving high reaction heat generation, so that a nitridation reaction of aluminum powder occurs almost simultaneously.
The present disclosure relates to a method of synthesizing aluminum nitride by the volumetric combustion synthesis, the latter method between the two types of combustion synthesis methods as introduced above.
According to the experimental results in a reference by Dian Zhang et al. (“Formation of Core-shell and Tadpole-like Structure in the Direct Nitridation of Aluminum Powder by N2 and NH3”, Journal of Alloys and Compounds, vol. 547 (2013) pp 91-99), when pure aluminum powder having a particle diameter of 53 to 120 μm is heated under a nitrogen gas atmosphere, the powder is all converted to aluminum nitride at a temperature of 1,400° C. or higher. Industrially, in order to obtain an aluminum nitride product without any residual aluminum via the direct nitridation method using a nitrogen gas, it is reported that a reaction must be performed at a high temperature of about 1,500° C.
Hereinafter, prior arts published in relation to synthesis of aluminum nitride via the direct nitridation method will be reviewed briefly.
U.S. Pat. No. 5,837,633 disclosed that aluminum nitride can be synthesized even at a low temperature of 550 to 660° C. when a formed body, in which 0.16 g of aluminum powder is pressed to a thickness of 0.75 mm and a diameter of 12 mm, or 6 g of aluminum powder is packed in a depth of 1 cm in a graphite tray having a length of 3 cm and a width and pressurized to 10 to 100 atmospheres with a nitrogen gas.
Okada et al. (T. Okada et al., “Direct Nitridation of Aluminum Complexes at Low Temperature,” Journal of Materials Science, vol. 38 (2000) pp. 3105-3111.) studied a nitridation reaction of a disc having a diameter of 12 mm and a thickness of 0.8 mm and with a relative density of about 65%, formed using aluminum powder with an average particle diameter of 24 μm, which occurs on heating the disc to 500 to 700° C. under a pressurized nitrogen gas atmosphere of 0.5 to 7 MPa after charging it into a furnace. It was reported that the aluminum was fully converted into aluminum nitride when a sample was pre-heated to 580° C. in vacuum prior to nitridation, and then nitridation-treated at nitrogen gas pressure of 4 MPa.
U.S. Pat. No. 6,159,439 discloses a method of synthesizing aluminum nitride by heating, at a temperature of 500 to 1,000° C. under a nitrogen gas atmosphere, a powder mixture containing 50 to 97 wt % of aluminum powder and 3 to 50 wt % of a nitridation promoting element including no more than 0.5 wt % of magnesium or an aluminum alloy thereof (for example, Al—Mg), based on total 100 wt % of the powder mixture. Here, coarse aluminum powder having a particle size of 210 μm or more is used, and finer powder of a magnesium-containing element or Al—Mg powder mixture is used as the nitridation promoting agent. Also, U.S. Pat. No. 6,159,439 discloses a method of performing nitridation treatment at a temperature lower than the melting point of aluminum, and then performing nitridation treatment again at a high temperature equal to or higher than 750° C.
A method of adding fine aluminum nitride powder to raw material of the reaction, which does not participate in the nitridation reaction of aluminum powder, may also be used to promote nitridation of aluminum powder via the combustion synthesis reaction in the thermally explosive mode. Here, the added aluminum nitride powder absorbs heat generated during the nitridation, thereby suppressing overheating and lowering a temperature of nitride formation. As a result, agglomeration of the aluminum powder is prevented, and the nitridation is enhanced. (M. Uda et al., “Preparation of Mixed Ultrafine (Al—AlN) Powders and Their Nitridation”, in Physical Chemistry of Powder Metals Production and Processing, pp. 261-269, October 1989, The Minerals, Metals & Materials Society, PA, U.S.A)
In U.S. Patent Application No. 2003/0099590 and U.S. Pat. No. 7,022,301, Miura et al. disclose a method of synthesizing aluminum nitride by heating a mixture of aluminum powder having an average particle size of 10 to 200 μm and home-made −100 mesh aluminum nitride powder at 460° C. for at least 10 minutes to form aluminum nitride on a surface of the aluminum powder, and then inducing nitridation of un-reacted aluminum at 500 to 1,000° C. under a nitrogen gas atmosphere pressurized to 80 to 300 kPa.
As described above, in order to synthesize aluminum nitride via direct nitridation by using aluminum powder as raw material powder, a method of admixing aluminum nitride has been used and to synthesize nitride a nitrogen gas atmosphere higher than atmospheric pressure has been used.